The GW ringdown, where the final object relaxes to a stable state, contains key information about the perturbed compact object's externally observably quantities. Recently, it has been argued [1]}, [2]} that the late-time ringdown signal may incorporate signatures for the existence of ECOs, such as wormholes, or horizon-scale quantum corrections [3]}, [4]}, [5]} (though see [6]}), in the form of echoes.
